Numerous occular conditions are known which result in the two eyes of an individual actually aiming at different points in space when the individual attempts to fixate on a target. Strabismus, or wall-eye, for instance, may be caused by various factors, including a relative weakness of one eye muscle, or the anomalous attachment or dimension of an eye movement muscle. As a diagnostic indicator of such underlying conditions, it is useful to measure the angle of deviation of the affected eye. This deviation is usually expressed in terms of its horizontal and vertical components, termed the horizontal (respectively, vertical) squint angle.
One conventional method of ascertaining that the squint angle is not zero, which is the desired normal condition, is to have the subject fixate at a distant target and then, while closely observing an eye of the subject, cover the other eye. If the uncovered eye jumps to a new position, it was previously aimed at a position offset from the target. This cover test method requires the diagnostician to observe closely the eye during a fraction of a second; the test determines the existence of a squint angle, but not its amount. A cover test using also a graded sequence of prisms may be used to quantify the deviation. A clinical method for quantifying an amount of angular deviation is the use of an amblyoscope, in which each eye separately visualizes an image, the image presented to one eye being offset via an optical path of adjustable angle from the image presented to the other eye. The optical path is adjusted until the two images coincide, with the horizontal offset of the path providing a measure of the relative angular displacement of the two eyes.
Another, non-quantified, technique for determining the existence of a squint angle has been practiced by J. Lang, reported in Mikrostrabismus. Vol. 62, Bucherei des Augenarztes, 2nd ed., Stuttgart: Enke Verlag, 1982. In this technique, the eyes of the subject are illuminated from above with a photoflash as a frontal photograph is made of the two eyes, so as to show the first and fourth Purkinje images, which are seen as a pair of small spaced-apart dots visible through the pupil. One eye is then covered and a second photograph taken. If the Purkinje images of the second photograph have changed position, this indicates that the eye has re-adjusted its aim after covering of the first eye. The method of Lang essentially derives from the work of M. Tscherning in the 19th century, who observed the first and fourth Purkinje images from directly in front of an eye illuminated from above. Tscherning used such observations, reported in his Optique Physiologique, Paris 1898, to determine the difference between the optical axis and the fixation line of the eye.
More recently, numerous researchers have utilized images of the first and fourth Purkinje images as a non-invasive means of tracking movement of the eye itself. Among such efforts are the devices shown in U.S. Pat. Nos. 3,712,716; 3,724,932; and 3,804,496 all issued for inventions of Tom N. Cornsweet and Hewitt B. Crane. Those patents show generally an instrument for imaging the first and pair of split-field intensity discriminating photo cells to control a tracking mirror, so as to develop signals indicative of the actual eye movement. Related devices are shown in U.S. Pat. Nos. 4,287,410 and 4,373,787, both issued to Hewitt D. Crane and Carroll M. Steele.
It is also known in the art to utilize a light source and a video camera to image the human eye for measuring or other clinical purposes, as in U.S. Pat. Nos. 3,598,107 of Ishikawa et al.; 4,257,688 of Matsumura; and 4,370,033 of Kani et al.